Alternating current or AC power systems established and operated by electric utilities and by many large industrial installations must be protected against electrical faults through the use of protective relay schemes located in appropriate power substations. These protective relay schemes clear power system faults by tripping circuit breakers and other devices to interrupt fault currents fed by the AC power system into the fault. The primary source of energy for operation of interrupting devices is a substation direct current or DC system, which normally consists of a bank of stationary batteries, connected in series to give an overall bank voltage typically in the range of 125 to 140 VDC, and a battery charger, which continuously charges the battery bank and which supplies DC current to normal station DC loads. So long as AC station service voltage is supplied to the charger input, the charger will supply nearly all of the DC current required for tripping the interrupting devices. However, if the fault results in a loss of the AC feed to the charger input, then the battery bank must supply all of the DC current required for tripping. If an interconnection between any two battery cells of the bank has opened (as a result of corrosion of interconnection hardware, for example), then the battery bank will not be able to supply the DC current necessary for operation of tripping devices to clear a fault. This is an open battery bank condition and it can result in catastrophic consequences for AC power system equipment.
Present alarm schemes to detect abnormal conditions of the DC system typically include a means of detecting whether the voltage of the DC system is either lower than a specified low limit, typically about 120 VDC, or higher than a specified high limit, typically about 145 VDC. This detection function is commonly achieved through the use of a voltage sensor(s) and over- and under- voltage relays (and associated electronic circuits) connected in parallel with the charger's DC output circuit. However, should the battery bank open with the charger continuing to operate, there would be no indication of this condition, since the DC system voltage would be maintained at normal levels by the charger. The over- and under- voltage relays would not sense that the battery bank had opened. There would be no charging current present in the interconnecting cables of an open bank, but voltage relays cannot detect this condition.